1. Field of the Invention
The present invention relates to an electromagnetic relay which opens and closes a contact point part using magnetic force generated when power is distributed to a coil.
2. Description of the Related Art
As shown in FIG. 1, an electromagnetic relay 9 is incorporated in a power circuit 50 for switching a current flowing in a power circuit 50 in the conduction state or a cut-off state in order to drive a travelling motor in vehicle such as a hybrid vehicle, an electric vehicle etc. Moreover, a fuse functional part 40 which shuts off the current by disconnecting due to Joule heat when unusual large current flows is also incorporated in the power circuit 50.
The electromagnetic relay 9 and the fuse functional part 40 are wired in series mutually. By shutting off either, power distribution in the power circuit 50 is stopped.
In FIG. 1, a reference symbol 51 indicates a high-voltage battery, a reference symbol 52 indicates a rotating electric machine made of a three-phase AC travelling motor, a reference symbol 53 indicates an inverter which converts DC power (AC power) to AC power (DC power). Here, the fuse functional part 40 is incorporated inside the high-voltage battery 51.
As shown in FIG. 2, the electromagnetic relay 9 has a contact point part 93 comprising a movable contact point 931 which is opened and closed by magnetic force generated by a coil 92 and a fixed contact point 932. A pair of the fixed contact points 932 is held by the fixed holder 934, and a pair of the movable contact points 931 is held by the movable holder 933 so as to mutually short-circuit.
In the above conventional electromagnetic relay 9, when power is distributed to the coil 92, the contact point part 93 is closed by magnetic force generated by the coil 92 whereby a conduction state is formed. On the other hand, when power is not distributed to the coil 92, magnetic force generated by the coil 92 disappears. Thereby the contact point part 93 is opened and a cut-off state is formed.
In such a configuration, when abnormality occurs in the power circuit 50 shown in FIG. 1, power distribution to the power circuit 50 can be interrupted by sending a signal to the electromagnetic relay 9 for making the electromagnetic relay 9 in the cut-off state.
Meanwhile, when the large current beyond a predetermined value flows, power distribution to the power circuit 50 can be interrupted by fusing the fuse functional part 40 by Joule heat.
However, when the electromagnetic relay 9 shifts to the cut-off state from the conduction state, an arc 8 may be generated in the contact point part 93 as shown in FIG. 3. For extinguishing the arc 8, an electromagnetic relay is disclosed in a Japanese Patent Application Laid-Open Publication No. 2005-347116, where the electromagnetic relay has arc-extinguishing magnets (not shown in the drawing) disposed lateral to the contact point part 93 so as to sandwich the contact point part 93.
However, in case that the current which was flowing in the electromagnetic relay 9 before being interrupted is a large current, it is difficult to extinguish the arc 8. Moreover, if the arc 8 continues too long, the arc heat will have a bad influence on surrounding parts. That is, there is an upper limit to the current value which can be interrupted in the electromagnetic relay 9. In order to raise the upper limit, it is necessary to enlarge distance H between contact points in the cut-off state. In this case, since larger magnetic force is required when switching the electromagnetic relay 9 from the cut-off state to the conduction state, the coil 92 is required to be larger. Consequently, the electromagnetic relay 9 is also required to be larger.
On the other hand, the fuse functional part 40 is disconnected by receiving a predetermined heat quantity. If the current value which can fuse the fuse functional part 40 is set low, the fuse functional part 40 may be disconnected even at the normal time. Therefore, it is necessary to set the fuse functional part 40 so that it may be disconnected when an unusual large current, far exceeding the current value which can flow at the normal time, flows.
That means, in the case that a large-sized electromagnetic relay is not adopted, there will be a current value which is not only larger than the upper limit which can be interrupted in the electromagnetic relay 9, but also smaller than the unusual large current, which can be interrupted in the fuse functional part 40, far exceeding the current value which can flow at the normal time. In other words, there is a current range which cannot be interrupted in either the electromagnetic relay 9 only or the fuse functional part 40 only. For this reason, there is a problem that it is difficult to manage an abnormality occurring while the current in this range flows.